Thermosettable resins are well known for their use in structural adhesives, in high performance composites, and in prepregs. Composites and prepregs made from these thermosettable resins and high strength fibers such as glass, ceramics, carbon and the like provide articles having considerably less weight than the same article made from metal. The articles, however, made using these thermosettable resins have been brittle, a characteristic that necessarily seriously limits their use. Also, because the viscosity of the thermosettable resins has been either too high during processing or too low during curing many of the articles are rejected because of the development of voids and imperfections in the articles.
Efforts have been made to control the viscosity of the thermosettable resins. Flow control agents, such as fumed silica, clay, whiskers, and high molecular weight polymers, have been added to thermosettable resins. These agents increase the viscosity over all temperature ranges, often making processing difficult or impossible and frequently harm the physical characteristics of the cured article.
High performance thermosetting resins include such resins as epoxies, bismaleimides, and cyanates. These resins react with curing agents or in the presence of polymerization catalysts to yield high performance resins that have gained wide acceptance as protective coatings electrical insulations, structural adhesives, and as a matrix resin together with reinforcing filaments in advanced composites. Where high glass transition temperatures, thermal resistance, and chemical resistance has been attained by the cured resin, the cured resin has generally had poor physical characteristics and been brittle, had poor impact resistance and fracture properties.
There are many references in the art teaching means for improving physical characteristics of cured thermosetting resins, primarily thermosetting epoxy resins, by use of a particular hardener and/or the addition of another polymer and/or an inorganic particulate material.
References among many others that teach means for improving the physical characteristics of cured thermosetting epoxy resin by the addition of a thermoplastic resin include the following:
U.S. Pat. No. 3,784,433 discloses that composites of unidirectional carbon fibers and a thermosettable resin are made by applying the resin as a continuous film to the carbon fibers, applying heat and pressure so that the resin flows about the fibers and forms a coherent structure, and then converting the resin into its solid, fusible B-stage. Heating cycles can be very short as there is no solvent to evaporate, and the thin layers can be cooled quickly.
U.S. Pat. No. 4,524,181 discloses compositions curable to articles having reduced susceptibility to mechanical and thermal shock comprising an epoxy resin and colloidally dispersed elastomeric particles.
U.S. Pat. No. 4,558,078 discloses compositions useful as a prepreg comprising an epoxy prepolymer and curative and optionally a second resin that can be present homogeneously or in the form of an interpenetrating network.
European Patent Application 0 274 899 discloses a prepreg consisting of a fiber reinforced resin having as a separate phase fine thermoplastic or thermosetting resin particles distributed throughout the prepreg, preferably the particles are localized in the inter-layer zones of the composite material and remain as a separate phase upon curing the prepreg.
In U.S. Pat. No. 4,157,360 there is disclosed melts of dicyanates and thermoplastic polymers and in U.S. Pat. Nos. 4,745,215 and 4,780,507, there are disclosed blends of dicyanates and thermosetting or thermoplastic polymers. In European Patent Application 0 301 361 there are described rubber-modified cyanate ester resins and polytriazines derived therefrom and in U.S. Pat. No. 4,804,740 there are described curable cyanate ester formulations containing thermoplastics. In U.S. Pat. No. 4,468,497, there is disclosed bis-imides, crosslinking agents, and compatible elastomers; the compositions of these references being useful for making prepregs.
M. T. Blair et al. at the 33rd International SAMPE Symposium of Mar. 7-10, 1988, presented a paper titled "The Toughening Effects of PBI in a BMI Matrix Resin" in which they disclosed the dispersion of 10% polybenzimidazole into a blend of bismaleimide and o,o'-diallylbisphenol A. Such a composition would not exhibit a "dissolution temperature" required in the composition of the instant invention.